25 MAXWELL'S LAW 



49 



prference only to the molecular motion in which all the 

 /components of the molecule equally take part. For the 

 atomic motions that exist by its side, the motions, that 

 is, which the atoms separately possess within the mole- 

 cule, the law can hold only with a certain limitation, since 

 there is a difference, which we shall discuss in 57 and 

 60, between the free motion of a molecule and the con- 

 strained motion of the atoms that is due to affinity. 



Maxwell's law needs also modification when the gas is 

 subject to external forces, such as gravity. We may here 

 neglect this action, as it does not come into account 

 in physical researches, but only in meteorological investi- 

 gations ; it is therefore sufficient to mention that, in 

 addition to Maxwell 1 himself, there are, among others, 

 ^Boltzmann, 2 Loschmidt, 3 and Ferrini 4 who have 

 P occupied themselves with this extension of Maxwell's law. 



26. Fuller Explanation of Maxwell's Law 



According to Maxwell's law the occurrence of the 

 zero value for one of the three components of velocity is 

 more frequent than that of any other given value. One 

 might be inclined, therefore, to conclude that the most fre- 

 quently occurring case would be that in which each com- 

 ponent, and consequently the resultant velocity, is vanishingly 

 small. This conclusion, however, must be false, as it can 

 only extremely seldom happen that a molecule comes to 

 rest in the midst of a swarm of molecules rushing rapidly 

 about. 



It is easy to explain the apparent contradiction between 

 Maxwell's law and this undoubtedly true fact. When 

 one of the components is zero there is no necessary reason 

 for the other two to vanish, but they may have any 

 possible value. In the case, therefore, which, according to 

 Maxwell's theory, is the most probable, one of the three 

 components may very well vanish without the resultant 



1 B. A. Beport, 1873, p. 29. 



2 Wiener Sitzungsber. Ixxii. 2. Abth. 1875, p. 427. 



3 Ibid. Ixxiii. 2. Abth. 1876, pp. 128, 366. 



4 Eendiconti d. E. Istituto Lombardo [2] xviii. 1885, p. 319. 



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